# File lib/needle/interceptor.rb, line 149
    def doing( &block )
      if @doing
        raise InterceptorConfigurationError,
          "you cannot redefine 'doing' behavior"
      end

      if @with
        raise InterceptorConfigurationError,
          "cannot specify 'doing' after specifying 'with'"
      end

      if block.nil?
        raise InterceptorConfigurationError,
          "you must specify a block to 'doing'"
      end

      @doing = block
      self
    end